ball joint spacers - ackerman changes

[Spoolin6er]

So I'm looking into my options for roll center correction but I do not want to add any ackerman (I'm actually trying to reduce it as much as possible, but that's another topic). I'm not an expert on this, so please correct me where I go off...

Now all the ackerman examples I've ever seen show a top view with the steering axis as a simple point (representing a vertical axis) making it very easy to draw a line through the ball joint and tie rod joint to get an idea of the ackerman percentage. However here, in the real world, we have a steering axis that is inclined so things seem to get complicated. In my searching I ran across this picture on zilvia:



This picture shows the problem with running lower ball joint spacers. However, to me it seems over exaggerated. The picture seems to suggest that the upright pivots about the ball joint on an axis parallel to the tie rod end. But it should actually pivot around the steering axis, which is draw through the lower ball joint and the strut top, right? So the critical dimension should be the perpendicular distance from the steering axis to the tie rod end.

This distance IS changed by spacing the lower ball joint because as it moves toward the outside of the car, the steering axis changes. (Makes me wonder what other effects on suspension geometry this has - scrub radius, bump steer, ect) But I expect the axis movement to be fairly small since the vertical seperation between the ball joint and the strut top is much greater than the horizontal shift of the ball joint.

I have not gotten too familiar with the underside of my 240 (yet) but the steering axis isn't as steep as the lower ball joint angle right? Why did nissan put this angle here in the first place?

All this to say, do you think the ackerman change due to ball joint spacers is really as large as people make it out to be? I really just need to man up and model the front suspension, but its just sooo much work... has no one draw it up?

[Def]

If you want to get ackerman out (drifting?), then it will add some. As to how it feels for track driving, the roll center correction dominates it all so the car feels like you up'd the spring rates and gained some traction on the outside front due to less roll.

Spacing down the tie rod increases ackerman slightly as well, but don't tell the drifto boys that.

I don't think the steering axis is as steep as the ball joint angle, but I don't think it's that far off in a stock car. It probably gets closer to the steering axis as the suspension compresses as well.

[josh18_2k]

If you're desperate for reduced Ackerman you can heat he spindle and twist the tie rod mount. There's a right way and a wrong way to do it, so definitely research lol. Ive seen this done on race cars

[Spoolin6er]

If you want to get ackerman out (drifting?), then it will add some....

shhh... - of course not, I just want to reduce scrub in the high speed sweepers

I guess I'm curious how much some is? Any good ways of measuring steering angles? If so we could compare the difference between the right and left wheels at different #turns of the steering wheel. I'm stock everything right now, so it would be interesting to see the change... I would have already measured it, but I cant think of a good, reasonably accurate way to.

[logr]

Move both joints down as a unit to negate the problems. You can also change the ackerman at the same time.

[Spoolin6er]

Move both joints down as a unit to negate the problems. You can also change the ackerman at the same time.

I guess the only way to do that is to cut space and weld the upright because spacing them down the same amount will increase ackerman as described above...

But anyone have any good ideas on ways to measure ackerman at home?

[josh18_2k]

wheel straight, draw lines on the ground. turn wheel, draw lines onthe ground. measure angles for inside and outside tires. do math?

[Spoolin6er]

This seems easier said than done. I can't think of any good ways to draw a line parallel to the wheel and one parallel to the chassis with any sort of accuracy. Maybe I'll check out how those crazy string alignment guys do things... It always amazed me that they could get a good, consistent alignment that way.